Complementary metal-oxide semiconductor (CMOS) image sensors are gaining in popularity over traditional charged-coupled devices (CCDs). A CMOS image sensor typically comprises an array of picture elements (pixels), which utilizes light-sensitive CMOS circuitry to convert photons into electrons. The light-sensitive CMOS circuitry typically comprises a photodiode formed in a silicon substrate. As the photodiode is exposed to light, an electrical charge is induced in the photodiode. Each pixel may generate electrons proportional to the amount of light that falls on the pixel when light is incident on the pixel from a subject scene. The electrons are converted into a voltage signal in the pixel and further transformed into a digital signal which will be processed by an application specific integrated circuit (ASIC) or an image signal processor (ISP).
A CMOS image sensor (CIS), or simply a CMOS sensor, may have a front side where a plurality of dielectric layers and interconnect layers are located connecting photodiodes in the substrate to the peripheral circuitry. A CMOS sensor is a front-side illuminated (FSI) image sensor if the light is from the front side of the sensor, otherwise it is a back-side illuminated (BSI) sensor with light incident on the backside. For a BSI sensor, light can hit the photodiode through a direct path without obstructions from the dielectric layers and interconnect layers located at the front side. This helps to increase the number of photons converted into electrons, and makes the CMOS image sensor more sensitive to the light source.
The image sensor market is being driven toward low cost, high image quality, and small camera module size. In digital still camera (DSC) approach, an ISP and a CIS are produced as two separated packages which occupy a larger board space and consume higher system power. Meanwhile, the system on chip (SOC) CMOS image sensor compact camera module (CCM) approach used in mobile handsets is facing the challenge of inadequate image data transmission rate, since the current image sensor applications require a standalone and powerful ISP. Although existing image sensor devices and methods of fabricating image sensor devices have been generally adequate for their intended purposes, as device scaling down continues, they have not been entirely satisfactory in all respects.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale.